This invention relates in general to telecommunications systems and more particularly to a DC to AC power converter for use as a ringing generator in telephone subscriber line circuits.
Ringing generators have been used for signaling calls between subscriber telephone instruments since the hand crank magneto generator of Alexander Graham Bell's day. In fact, magneto generators powered by -50 volt DC motors are still used for signaling calls today.
Today however, there is a need to reduce the power consumption of large telecommunication systems and therefore a need for increasing the efficiency of ringing generators.
To ensure continuity of service within a telecommunication system, all ringing generators are powered from a DC battery source. A battery voltage of -50 volts is generally used as the input voltage and an AC voltage of 85 to 100 volts RMS is generated at the output. The most common battery voltage is -50 volts for local subscriber lines and trunk circuits. Carrier systems for long distance toll communications will typically use +130 volts and -130 volts to power repeaters along the carrier span. The +/-130 voltages are usually derived from DC to DC converters powered from the -50 volt battery.
For example, current subscriber line circuitry powered by carrier spans that use repeater links are powered by the aforementioned +/-130 volts with the current limited to 200mA. Therefore, the power consumption of the subscriber line circuitry at the remote terminal is strictly limited. Since the ringing generator is the most power hungry piece of equipment at the remote site there is substantial interest in developing a ringing generator which is highly efficient.
One efficient ringing generator presently known is the suppressed carrier double sideband generator. This ringing generator uses a microprocessor which contains a look-up table. The converter uses the data in the look-up table to create a sinusoidal output. A digital-to-analog converter is used to create a dynamic signal which controls a pulse width modulator. The pulse width modulator has two phases which are controlled by the microprocessor. The phase information is fed through an isolation transformer where the original AC waveform is reconstructed by using a synchronous detector and filter. This particular ringing generator can achieve efficiencies of 85% however, these efficiencies are gained at the expense of complexity (and thus reliability) as well as cost. Additionally, the timing of the microprocessor, switches and synchronous detector of this converter is quite critical, and can present a number of design problems.
Another efficient ringing generator presently known is the Cuk converter. The Cuk converter uses a much simpler converter topology than the suppressed carrier double sideband generator described above and has attained efficiencies of over 90%. The Cuk converter includes a transistor which is fed a pulse width modulated square wave that varies according to the amplitude of the sine wave that the converter is trying to simulate. The disadvantage to this type of converter is the large size of the capacitors necessary, (typically 4,700uF or larger) to achieve high efficiencies. The Cuk converter is considered a capacitive energy transfer converter and the value of the capacitors is intrinsic to its design.
It therefore becomes the object of the present invention to provide a new and efficient means for generating ringing voltages to subscriber lines without any of the disadvantages mentioned above.